1. Field of the Invention
The present invention relates to a reference apparatus used for determining a current or a voltage of a semiconductor device such as a floating gate type non-volatile memory, a method of manufacturing such a reference apparatus, and a verifying method using such an apparatus.
2. Description of the Related Art
In a floating gate type non-volatile semiconductor memory device, a current flowing in a cell under predetermined conditions and a current flowing to a reference transistor are compared by a differential amplifier so as to determine cells in an array by "0" and "1". This reference transistor is called reference cell.
Such a reference cell is generally formed to have the same structure as that of an array cell for the purpose of maintaining a similarity with respect to the cell in array, or into a so-called dummy cell type in which the control gate and the floating gate are connected to each other. In short, a reference cell array serves as a miniature version of cell array.
Also, in a reference which determines an output of a logic device, the reference level is generally limited to the threshold value of the transistor used in the logic device.
Recently, in accordance with divergence in the characteristics of devices, it becomes necessary to make determination in terms of "0", "0.5" and "1" as in the case of, for example, a multi-value memory.
With a conventional approach, in reply to such a demand, a plurality of sense amplifiers having different sense ratios are used to deal with the divergence of devices, that is, a multiplicity in value.
However, with this approach, the circuit area is increased, thereby increasing the chip area and the cost.
For verification in which the threshold value of the memory cell after programming or erase is confirmed, the non-volatile semiconductor memory device changes Its verifying voltage in accordance with the state to be verified. However, when a verification is carried out in the above-mentioned manner, the voltage supply circuits are inevitably complicated.